An analog electrical circuit, including both resistances and capacitances for roots, stem, and leaves, was developed for a desert leaf-succulent,Agave deserti. Based on soil water potentials and observed transpiration rates and the influence of leaf, stem, and root relative water content on the water potential of each organ, water potential could be predicted at various locations in the transpiration stream. Simulations indicated that as stomata opened at night for this crassulacean acid-metabolism plant, most of the water initially transpired came from the leaves. Water movement from the soil was greatest throughout the night and the early morning; the maximal water uptake rate from the soil was only about half of the maximum transpiration rate. As drought increased, lower transpiration rates led to smaller water losses, in turn leading to progressively earlier recharge of the leaf capacitance. The substantial and predictable daily changes in plant water potential may have profound effects on physiological processes in the cells near the transpiration stream.
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